feat(theme): App 初始化时设置 data-theme 属性，首次渲染即为主题

frontend/src/App.tsx

@@ -2,14 +2,20 @@ import { useEffect } from 'react'
 import { RouterProvider } from 'react-router-dom'
 import { router } from './router'
 import { useAuthStore } from './stores/auth'
+import { useThemeStore } from './stores/theme'
 import './styles/global.css'
 
 export default function App() {
   const loadFromStorage = useAuthStore(s => s.loadFromStorage)
+  const theme = useThemeStore(s => s.theme)
 
   useEffect(() => {
     loadFromStorage()
   }, [loadFromStorage])
 
+  useEffect(() => {
+    document.documentElement.setAttribute('data-theme', theme)
+  }, [theme])
+
   return <RouterProvider router={router} />
 }